The effects of the pineal gland on pituitary gland function are investigated by determining the effects of pineal products on the LH-response of neonatal rat pituitary glands to treatment with LH-releasing factor. It has been discovered that melatonin can block this response and that it acts rapidly through a relatively specific mechanism at low concentrations. The effects of melatonin have been observed in organ culture and in vivo. Efforts to synthesize a long acting melatonin analog have produced 4,6-difluoromelatonin and 6-difluoromelatonin. These compounds were found to be as potent as or more potent than melatonin. In addition to melatonin, other pineal products including serotonin, methoxytryptamine, and pineal peptides were found to be active in blocking the effects of LRF on the pituitary gland.